U.S. Pat. No. 3,852,045 to Wheeler, Sump, and Karagianes discloses a porous metallic material including a network of interconnected voids or pores. It is formed on a surgical prosthetic device for tissue ingrowth purposes. The voids or pores about the resulting surface areas are produced in the coating material by use of a composite material including expendable void formers. The described composite material is treated by high energy rate forming pressures to densify its structure prior to removal of the expendable void former. Substantial thicknesses of the void coating on substrate metallic elements is disclosed.
While the products resulting from the systems taught in U.S. Pat. No. 3,852,045 have performed satisfactorily, the practical application of the system is severely limited by both the expense and availability of equipment for the required high energy rate forming steps. Furthermore, such steps are of questionable value when attempting to produce a relatively thin porous coating on implant elements, since the high pressures to which the elements would be subjected might result in structural damage to them.
U.S. Pat. No. 3,986,550 to Restaker et al describes prosthetic devices having porous sections. It discusses prior efforts to use consolidated metal powders to produce porous metals for this purpose, but dismisses them as being brittle and having unacceptable toughness. The patent specifically describes a process for producing a porous section by use of short fiber strands. The strands are molded and sintered to interconnect the metal fibers. It states that by repressing procedures, external dimensions of the coated prosthesis can be precisely regulated to the excavation in the receiving bone so that a zero clearance fit is achieved. It describes that long wire lengths give more interlock and better molded strengths, but notes that the longer the wire, the more difficult it is to feed into dies. No mention is made of preforming the wires or using wire mesh of any type.
An article published in the Journal of Bone and Joint Surgery, Volume 53-A, No. 1, January, 1971, Pages 101 through 114, titled "Sintered Fiber Metal Deposits as a Basis for Attachment of Implants to Bone", by Galante et al, also describes the molding and sintering of short metal fibers for production of implant materials. It contains a review of available literature publications relating to porous elements of this type, including open pore materials derived from powders consolidated and shaped in molding dies under pressure. The article discloses coating of cylinders by pressing fiber sleeves to form them separately, but the sleeves discussed are produced from individual kinked lengths of wires. This would appear to expose fiber ends at the resulting coated surface and would produce a surface that would include discontinuities and lack desired uniformity. Also, individual fibers pressed in a random mat would not result in a coating having uniform porosity or strength.
According to the present invention, relatively thin porous metallic coatings are produced about selected surface area configurations on a bone implant element by performing a sleeve or surface covering from a wire mesh produced from continuous lengths of wire. The mesh is preferably knitted, braided, wound or woven in some manner to produce a uniform mesh structure which, when pressed, will have relatively uniform porosity in the resulting voids that are formed between the wires. The wires are made from a material either identical to or metallurgically compatible with the metallic surface being coated. They are heated and pressed in place about the surface to effect diffusion bonding between engaged wire sections as well as between the surface area and the wire surfaces in contact with it.